Periods and fertility in the menstrual cycle - Periods

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The length of the menstrual cycle varies from woman to woman, but the average is to have periods every 28 days. Regular cycles that are longer or shorter than this, from 21 to 40 days, are normal.

"The menstrual cycle is the time from the first day of a woman's period to the day before her next period," says Toni Belfield, a specialist in sexual health information, and a trained fertility awareness teacher.

"Girls can start their periods anywhere from age 10 upwards, but the average is around 12 years," says Belfield. "The average age for the menopause (when periods stop) in this country is 50 to 55."

Between the ages of 12 and 52, a woman will have around 480 periods, or fewer if she has any pregnancies.

What happens during the menstrual cycle?

To understand the menstrual cycle, it helps to know about the reproductive organs inside a woman's body. These are:

2 ovaries – where eggs are stored, developed and released

the womb (uterus) – where a fertilised egg implants and a baby develops

the fallopian tubes – two thin tubes that connect the ovaries to the womb

the cervix – the entrance to the womb from the vagina

the vagina

The menstrual cycle is controlled by hormones. In each cycle, rising levels of the hormone oestrogen cause the ovary to develop and release an egg (ovulation). The womb lining also starts to thicken.

In the second half of the cycle, the hormone progesterone helps the womb to prepare for implantation of a developing embryo.

The egg travels down the fallopian tubes. If pregnancy doesn't occur, the egg is reabsorbed into the body. Levels of oestrogen and progesterone fall, and the womb lining comes away and leaves the body as a period (the menstrual flow).

When are you most fertile?

"Theoretically, there's only a short time when women can get pregnant, and that is the time around ovulation," says Belfield.

It's difficult to pinpoint exactly when ovulation happens but in most women, it happens around 10 to 16 days before the next period.

"It's not accurate to say that all women are fertile on day 14 of the menstrual cycle," says Belfield. This might be true for women who have a regular, 28-day cycle, but it won't apply to women whose cycles are shorter or longer.